In control systems of the kind mentioned in the introduction, with a plurality of parallel-working separately-excited motors, because of unavoidable tolerances in the motors these will not share the load equally, i.e. their armature currents will be mutually different. Usually, each individual motor drives its own wheel axle and unavoidable variations in the wheel diameters will then contribute to a non-uniform loadsharing between the motors. This entails an incomplete utilization of the theoretically possible tractive effort. A solution to this problem is proposed in Swedish Pat. No. 8600969-3. According to this patent, difference signals, produced by differences between the armature currents in a first and a second motor and by differences between the armature currents in the second and a third motor, are utilized. These difference signals are then used as control signals for changing the fields in the three motors, so that the armature currents therein become equally great. This results in a maximally available tractive effort.
One condition for a satisfactory functioning of the loadsharing system according to the above is, however, that no wheel axles overspeed; that is to say, that the wheels are running at a higher speed than the ground speed of the vehicle, the wheel axle is slipping. The armature current in the motor concerned is greater than what is permitted by the adhesion between the driven wheels and the base. Upon such slipping, that of the parallel-connected motors which has the lower armature current will sense a voltage stiff source. Its armature current and torque thereby fall rapidly, i.e. the motor loses armature current as a result of the overspeed. The loadsharing system attempts to maintain the armature current and thereby counteracts this desirable relief in connection with slipping. One way of solving this problem is proposed in Swedish Pat. No. 8305869-3. This patent proposes forming for each motor a detected overspeed which constitutes the difference between the speed of the motor and a reference speed. The overspeed is allowed to control a signal which corrects the loadsharing system so that the load of the respective motor is placed in a favourable position in a so-called adhesion diagram. This means that each individual wheel axle is maintained near the slipping limit. The difficulty with this overspeed feedback control is to sense the true speed of the vehicle. This forms the basis for calculating the overspeed of the motors.
A simple way of sensing the true speed presents itself if the vehicle is equipped with an unpowered axle. If this is the case, this axle runs at ground speed which is converted to the desired reference speed.
An additional alternative for achieving a transducer for the ground speed of the vehicle is to install a radar which measures the speed of the vehicle in relation to ground. Such a ground radar functions well under most conditions. However, it has turned out that the failure tolerances of the radar are too great if it is utilized as a reference for the overspeed feedback control when this is to limit overspeeds to a maximum of 3 km/h. The accuracy of the radar is influenced by acceleration, deceleration, mounting angle, curves, and bridges.
At too high overspeeds, problems with torsional oscillations in the wheel axles may arise. These problems arise under relatively good adhesion conditions. The adhesion diagram of the wheel axle then shows a negative inclination for the curve which indicates the tractive effort of the motor as a function of the overspeed.
As an example, it has been found that torsional oscillations may arise during six-axle slipping if at the same time the radar shows a somewhat too high speed in relation to the true ground speed. The overspeeds result in a loss of tractive effort, which in turn leads to the vehicle losing speed and causes the radar instantaneously to show a still higher speed compared with that of the vehicle because the radar equipment has a somewhat time-delayed reaction. All this leads to an even higher overspeed with an ensuing risk of, inter alia, torsional oscillations in the wheel axles.
Radar failure indications of the above-mentioned kind are common under certain conditions. This means that the problems cannot be solved by small adjustments of the radar equipment or in the system for overspeed feedback control.